For environmental reasons, in recent years, there has been an increasing trend to reduce the amount of all chemicals, including water, used in photographic processing. Fixing photographic materials is necessary to remove any undeveloped silver halide after development which would otherwise slowly print-out and become indistinguishable from the image. In addition to this primary function, fixers are traditionally required to perform a number of other roles. These include, stopping photographic development and playing a part in washing out or decolorizing some film or process components. In order to accomplish the first of these, fixers are made sufficiently acidic to rapidly quench the development reactions within the film being processed. Most of the other secondary functions of fixers are achieved by components of the fixer not specifically included for that purpose.
In the graphic arts industry, very high contrast black-and-white materials are used. Ideal graphic arts images are formed with areas of maximum density (black) and minimum density (clear for film and white for paper) only. Traditionally, the major requirement for the washing section of a processor has been to maintain low levels of retained fixing agent (e.g. ammonium thiosulfate) in the processed film. This has usually been achieved by using very high wash replenishment rates typically between 2 and 10 liters of water per square meter of film processed. Retained non-image silver has not usually been considered a major cause of image deterioration since fixer replenishment rates have also been high. Also graphic arts processors have sometimes been equipped with silver recovery systems which remove silver from the fixing solution and so maintain low silver levels, typically around 2 grams per liter of fixer. With such low silver levels in the fixing bath and with large dilutions of silver carried into the wash section made possible by the high wash replenishment rates, the control of retained non-image silver has not been a problem. However, with the recent trend to use less wash water and fixer, and if for any reason (for example, cost, convenience, or potential hazards) a fixer silver recovery unit is not considered desirable, the levels of silver in the wash baths will rise.
U.S. Pat. No. 3,828,172 (Schickler) describes a method and apparatus for controlling the replenishment rate of chemicals expended during processing of photographic materials whereby the replenishment rate is linked to a calculated image silver signal.
European patent 0,456,684 (Rider) describes a method of controlling the rate of replenishment of chemical solutions used in photographic processing wherein a signal related to the measured exposure given to the photographic material is used to control the replenishment rate.
Soluble complexes of silver with fixing agent are by-products from the fixing reaction. These complexes are produced in the photographic material as the fixing agent reacts with undeveloped silver in the form of silver halide. The complexes diffuse out of the material and into the bulk of the fixing solution. Without silver recovery on the fixing bath, the concentration of complexed silver may build up to quite high levels, especially when low replenishment rates are used for the fixer and when the level of silver in the photosensitive material is high. Since fixing rate shows an inverse dependence on silver concentration in the fixer bath, the time required to clear the film will also depend on the silver level. Whilst silver recovery is therefore beneficial for the performance of the fixer bath, it represents significant extra capital cost. I have now found that it is not absolutely necessary.
It is useful to distinguish between two types of chemical species found in seasoned photographic solutions. There are those whose concentrations are largely independent of average exposure given to the photographic material being processed. These may be referred to as "image-independent" species. The independence arises because either a relatively small percentage of the total amount contained within the solution is used by an image dependant mechanism (eg thiosulfate) or because the reactions responsible for consuming these species are not primarily concerned with an image dependant mechanism (eg a pH buffer or antioxidant). For graphic arts black-and-white materials an example of an image-dependent chemical is silver (as silver complexes).
A particular problem for graphic arts films is a rise in the optical density in the ultra-violet region of the spectrum of the non-image areas, referred to as "UV Dmin" upon ageing of processed film. Frequently, ultra-violet contact exposures are used to copy a graphic arts film onto a printing plate or another piece of film and very high contrast images are needed for accurate copying. If, due to ageing, the difference between the minimum and maximum optical density of the image to be copied is reduced, the contrast of the image is effectively lowered. When the image is copied, inaccuracies may result. Furthermore, if the minimum density of the image increases, the overall exposure time for the copying process increases. For other types of silver halide images, whether black-and-white or color, changes in the tone scale and contrast of the image upon ageing are also detrimental even if no further copying process is involved because the quality of the image is reduced.
It has been determined experimentally that the action of non-image retained silver is very significantly worse for image degradation, and in particular for UV Dmin increase, than that of an equal weight of retained fixing agent. Normally, silver complexes are present in the fixer and wash solutions at significantly lower concentrations than the fixing agent. In certain circumstances however, especially in processors without silver recovery, the control of residual silver in the processed film may become more important than the control of residual fixing agent in determining wash water requirements.